What is the duration of the type I lesion?

In contrast to the delayed Type IV contact dermatitis, Type I allergic reactions have a much shorter duration, usually approximately 20-30 minutes. In some instances the response can take up to an hour if it is a slow-reacting Type I hypersensitivity. This is a primary reason why Type 1 reactions are called immediate hypersensitivities and the others (Type IV) are called delayed.